Remove mem_alloc_error()

As thd->alloc() and new automatically calls my_error(ER_OUTOFMEORY)
there is no reason to call mem_alloc_error()

Other things:
- Fixed bug in mysql_unpack_partition() where lex.part_info was
  changed even if it would be a null pointer

sql/sql_partition.h

@@ -133,7 +133,6 @@ Item* convert_charset_partition_constant(Item *item, CHARSET_INFO *cs);
   @param[in]     table Table containing read_set and fields for the row.
 */
 void append_row_to_str(String &str, const uchar *row, TABLE *table);
-void mem_alloc_error(size_t size);
 void truncate_partition_filename(char *path);
 
 /*